Sunday, July 3, 2011

Physical design of notch-blend filter

A small Hammond box will have 4 pairs of RCA jacks. The inputs and outputs will be on the ends of the top plate, most widely spaced apart. Two pairs, slightly closer together, in the middle.

The inner pairs are control jacks for two possible blend circuits, only one will be implemented initially. When you plug shorting plugs into one middle pair, that blend circuit is activated. The active blend parts are strung between the "grounds" of the plugs. All plugs are actually isolated from ground. A separate ground lug on box portion connects to system grounding point.

To disconnect both blend circuits, shorting plugs can be removed entirely or plugged into alternate circuits. Since neither circuit would be completed, both would be deactivated.

The plugs used with be gold plugs from Dayton audio, 091-1120. I have tested, and unlike the claim of one reviewer, the ones in my possession are not attracted to magnet, even the tab washers. They come with white insulating washers which feel like teflon or polyethylene for isolation.

An external blend circuit could be plugged into any middle pair, with no connection to the jack ground, for convenient testing.